The radioactive contaminants removed from the metallic pipes in the ferric solution are now concentrated in the iron phosphate sludge, mainly in the form of phosphates. The sludge composition is suitable to directly obtain a phosphate glass by means of a proper thermal treatment at temperatures higher than 1000°C. Proper crucibles has to be used: due to the features of the sludge and the temperatures required, metallic crucibles could fail, while ceramic crucibles could be the best and cheapest choice.

During the heating of the sludge, there is a first loss of water at around 150°C and a second one at around 480°C. Dehydration is complete within 600°C. There could also be the loss of P₂O₅ for volatilization but to a limited extent. To avoid the formation of bubbles during melting, heating is divided in a first essication step at 125°C overnight (at least 16 hours), a second dehydration step at 625°C for at least 12 hours and finally the temperature is increased to 1100°C and kept for 2 hours to guarantee the homogeneity of the melted material. During the essication step, there is an increase of volume, of about 40%, that has to be taken into consideration. The cooling has to be fast, to avoid the formation of crystalline phases in the material, which could reduce the chemical stability of the final wasteform.

The glass so-obtained has a Fe to P molar ratio close to the value 0.66, that is the optimal value for an iron phosphate glass with the best chemical durability. During the whole thermal treatment, gas emissions have to be kept under control by a suitable filtration system in order to avoid the leakage of radioactive gaseous emissions.

The wet sludge containing the radioactive contaminants has to undergo the thermal treatment at high temperature in order to obtain the final glass wasteform. Set the right temperature profile to obtain the complete vitrification of the sludge and a glass of suitable properties.

Pay attention, you are working with radioactive materials!